Electrodeposition of bright zinc plate



United States Patent 2,129,602 ELEC'I'RODEPOSI'IION 0F BRIGHT ZINC PLA George Robert Van Houten, Greenfield, tun.

No Drawi g- Amilication July 29, 1952,

Serial No. 301,601

Claims. (Cl. 204--55) This invention relates to the art of electroplating and, more particularly, to the electrodeposition of bright zinc plate from an acid bath.

Heretofore, the typical proprietary bright zinc bath was based on a cyanide solution. The material and operating costs of employing a cyanide bath were relatively high and the health hazards accompanying the use of such bath were quite serious. In a plant employing not only cyanide baths but also acid baths, the cyanide baths had to be very carefully safeguarded to prevent the inadvertent addition of acid which would release the extremely dangerous hydrogen cyanide gas. For this reason, washing operations had to be extensive to prevent any acid carryover from pickling operations. If it were possible to plate bright zinc from an acid, instead of a cyanide bath, these diiliculties would be avoided, plating costs would be greatly reduced, and considerably higher current densities could be used than with cyanide baths. Although this problem existed in the art for some time and various suggestions and proposals were made to solve the same, none, as far as .I am aware, of these suggestions and proposals was completely satisfactory and successful for the electrodeposition of bright zinc plate from an acid bath on a practical and industrial scale.

I have discovered that the outstanding problem may be solved in a remarkably simple manner.

It is an object of the present invention to improve zinc plating processes.

It is another object of the inventionto provide a novel and improved process of electrodepositing bright zinc plate from an acid bath.

It is a further object of the invention to provide a novel bright zinc plating bath comprising an acid zinc solution in combination with an additive which eliminates the difiiculties and inconveniences connected with the use of conventional cyanide baths.

The invention also contemplates a novel bright zinc plating process employing an acid bath which is simple and inexpensive to practice on an industrial scale, is more nearly free from health hazards and is capable of producing a bright zinc deposit of high quality.

Other and further objects and advantages of the prescut invention will become apparent from the following description and from the appended claims.

Broadlystated, in accordance with my invention, bright deposits may be produced from an acid zinc bath by the addition of very small amounts of selenium or tellurium to the electrolyte, the preferred range of such addition being from 0.01 mg. to 20 mg. per liter. I have found that if the electrolyte has a concentration of 0.01 mg. to 20 mg. per liter of either selenium or tellurium, the deposit will be bright, the degree of brightness depending somewhat on the exact selenium or tellurium concentration. The selenium or tellurium may be added to the electrolyte as the element, the oxide, the acid or acid salt, or the sulfur compound (e. g., thio) homolog. Examples of such compounds are seleno phenol, CeHsSeH;

ethyl selenide, (CaHs) :Se; ethyl telluride, (C aI-IshTe; and

seleno urea, (NHahCSe. Use of the oxide is quite'satisfactory, but larger amounts of the thio type compounds can be used, provided the organic can be tolerated in the bath. The acid zinc plating bath may be any one of the conventional baths, such as, for example, a zinc sulfate solution which may also contain free sulfuric acid.

To assure consistently satisfactory results, it is, of course, desirable to maintain the concentration ofthe additives in the bath substantially constant. A simple, fast and inexpensive method of selenium or tellurium analysis for the concentration range contemplated by the invention not being available at the present time, maintenance of the proper concentration has to be based at least to some extent on deposit appearance and practical experience. Since the additive, selenium or tellurium, plates out or is otherwise removed in some consistent manner, a rate of removal may be determined]. This rate is a function of the cathode current density, concentra tion of the additive in the electrolyte, and, to a very slight extent, temperature. Once the rates are determined, ad-

gitions may be made on the basis of so much. per ampere our.

A considerably simpler automatic method of addition is possible by using anodes of a metal relatively insoluble in the electrolyte containing the additive in the form of aminor alloying ingredient so that upon gradual corrosion of the anode the additive is-introduceninto the bath at a desired predetermined rate. Anodes of this general type and theprocess of using the same in electrw plating are disclosed and broadly claimed in my co-pending application, Serial No. 301,602, filed concurrently with the present application. The anode metal is preferably lead which contains small amounts of selenium or tellurium. Anodes of this type will corrode: justrapidly enough to keep the addition agent or additive at or near the proper level, since anode corrosion rates vary with current density. As this method of introducing the additives of the present invention is partially self-adjusting, a fairly wide range of current densities is permissible without any special additions of the active agent. Furthermore, the small amount of lead introduced into the electities of the additive cause nodular deposits and near zero cathode current efficiencies. Efiective concentrations of selenium or tellurium in substantial excess of 20 mg. per liter of the electrolyte are certain to cause poor deposits at normal dePOSIUOH rates. If the selenium-lead or tellurium-lead alloy anode composition is much in excess of the eutectic, circa 0.005% by weight selenium or tellurium, the resulting additive concentration. in the electrolyte is certain to be too high at all except the highest cathode area to anode area ratios.

The baths preferred for the purposes of the present invention contain to 200 grams per liter of zinc, for example in the form of zinc sulfate, zero to 100 grams per liter of free sulfuric acid, and 0.1 to 2 milligrams of.

to retain their original brightness over long periods of Although the present invention has been disclosed in connection with a few preferred embodiments thereof, variations and modifications maybe resorted toby those skilled in the art without departing .from the principles of -the invention. Thus, as the presence of a small amount of lead in the electrolyte appears to be synergistic, some lead may be directly introduced into the electrolyte in case the anode is formed of a metal other than lead. I consider all of these variations and modifications to be within the true spirit and scope of the present invention,

as disclosed in the foregoing description and defined by the appended claims.

I claim:

l. A bright zinc plating' bath comprising an acid solution containing zinc ions, and 0.01 mg. to 20 mgJper liter of an additive selected from the group consisting of selenium and tellurium in said bath.

2. A bright zinc plating bath comprising an acid solution containing zinc ions, and 0.1 mg. to 2 mg. per liter of an additive selected from the group consisting of selenium and tellurium.

3. A'bright zinc plating bath comprising an acid zinc solution, free'acid, and 0.01 mg. to 20 mg. per liter of an additive selected from theg'roup consisting of selenium and tellurium.

4. A bright zinc plating bath comprising an acid zinc solution, and 0.01 mg. to 20 mg. of selenium per liter of said solution.

5. A bright zinc plating bath comprising an acid zinc solution, and 0.01 mg. to 20 mg. of tellurium per liter of said solution.

6. A bright zinc plating bath comprising an aqueous solution of zinc sulfate containing 100 to 200 grams per liter of zinc, zero to 100 grams per liter of free sulfuric acid, and 0.01 mg. to 20 mg. of an additive selected from the group consisting of selenium and tellurium.

7. The process of producing bright zinc plate which comprises providing an anode and a cathode in an acid zinc solution containing 0.01 mg. to 20 mg. per liter of an additive selected from the group consisting of selenium and tellurium, and passing an electric current between said anode and cathode thereby causing the deposition of bright zinc plate on said cathode.

.8. The process of producing bright zinc plate which comprises providing an'anode and a cathode in an acid zinc solution containing an additive. selected from the group consisting of selenium and tellurium, maintaining the concentration of said additive between 0.1 to 2 mg. per liter of solution, and passing electric current between said electrodes thereby causing the depodtion of bright zinc plate on said cathode.

9. The process of producing bright zinc plate which comprises providing ananode and a cathode in an acid zinc solution containing an additive selected from the group consisting of selenium, tellurium, the oxides, acids and acid salts of selenium and tellurium, and the sulfur homologs of the oxides, acids and acid salts of selenium and tellurium such as to maintain the vetfective concentration of the metal of the selected additive between 0.01 mg. and 20 mg. per liter of solution, and passing an electric current between said anode and cathode thereby causing the deposition of bright zinc plate on said cathode.

10. The process of producing bright zinc plate which comprises providing an acid zinc solution containing an additive selected from the group consisting of selenium and tellurium in a concentration between 0.01 mg. and 20 mg. per liter, introducing a lead anode containing the selected additive in an amount not over 0.005% by weight and a cathode into the solution, and passing an electric current between said anode and cathode to deposit bright zine plate on said cathode while maintaining the concentrationof the additive in the solution by the simultaneous corrosion of the anode.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Ser. No. 351,241, Weiner (A. P. C.), published May 18, 1943. 

9. THE PROCESS OF PRODUCING BRIGHT ZINC PLATE WHICH COMPRISES PROVIDING AN ANODE AND A CATHODE IN AN ACID ZINC SOLUTION CONTAINING AN ADDITIVE SELECTED FROM THE GROUP CONSISTING OF SELENIUM, TELLURIUM, THE OXIDES, ACIDS AND ACID SALTS OF SELENIUM AND TELLURIUM, AND THE SULFUR HOMOLOGS OF THE OXIDES, ACIDS AND ACID SALTS OF SELENIUM AND TELLURIUM SUCH AS TO MAINTAIN THE EFFECTIVE CONCENTRATION OF THE METAL OF THE SELECTED ADDITIVE BETWEEN 0.01 MG. AND 20 MG. PER LITER OF SOLUTION, AND PASSING AN ELECTRIC CURRENT BETWEEN SAID ANODE AND CATHODE THEREBY CAUSING THE DEPOSITION OF BRIGHT ZINC PLATE ON SAID CATHODE. 